A spark plug, which is used for igniting an internal combustion engine such as a gasoline engine, has a structure comprising an insulator provided on an outer side of a center electrode, a metal shell provided further outside thereof, a ground electrode that is attached to the metal shell and forms a spark discharge gap between itself and the center electrode. The metal shell is generally made of iron-based material such as carbon steel. In many cases, a surface of the metal shell is plated for corrosion protection. A technique that adopts, as a plating layer, a double-layered structure including a Ni plating layer and a chromate layer is known (Patent Document 1). However, inventors of the present application have found that corrosion resistance of a portion deformed during swaging for the spark plug is an important issue even when such a plating layer having two or more layers is adopted. Hereinafter, an exemplary structure of the spark plug and a process for swaging such a spark plug will be first described. Then, a portion of swaging deformation, which causes the issue of corrosion resistance, will be described.
FIG. 1 is a cross-sectional view illustrating a main part of an exemplary structure of a spark plug. The spark plug 100 has a cylindrical metal shell 1, a cylindrical insulator 2 installed in the metal shell 1 such that its tip portion projects therefrom, a center electrode 3 installed in the insulator 2 such that its tip portion projects therefrom, and a ground electrode 4, having one end being coupled to the metal shell 1 and another end being arranged so as to face the tip portion of the center electrode 3. A spark discharge gap g is formed between the ground electrode 4 and the center electrode 3.
The insulator 2 is made of, for example, ceramics sintered body such as alumina and aluminum nitride and has, in its inside, a through hole 6 for installing the center electrode 3 along the axial direction of the insulator 2. A terminal metal piece 13 is inserted into and fixed on the side of one end of the through hole 6. The center electrode 3 is inserted into and fixed on the side of the other end of the through hole 6. A resistor 15 is provided between the terminal metal piece 13 and the center electrode 3 in the through hole 6. Both ends of the resistor 15 are electrically connected to the center electrode 3 and the terminal metal piece 13 through conductive glass seal layers 16, 17, respectively.
The metal shell 1 is made of metal such as carbon steel and is formed in a hollow cylindrical shape. The metal shell 1 serves as a housing of the spark plug 100. Formed on an outer periphery of the metal shell 1 is a thread portion 7 for attaching the spark plug 100 to an engine block that is not shown. It should be noted that a hexagon portion 1e serves as a tool engagement portion for engaging a tool such as a spanner and a wrench when attaching the metal shell 1 to the engine block and has a hexagonal cross-sectional shape. A linear packing member 62 is arranged on a rear-side periphery of a flanged projecting portion 2e of the insulator 2, which is located between an outer surface of the insulator 2 and an inner surface of an opening of the metal shell 1 on the rear side (upper side in the figure). A filled layer 61 such as talc and a ring-shaped packing 60 are arranged in this order on the further rear side of the linear packing member 62. In an assembling process, the insulator 2 is pushed toward the front side (lower side of the figure) of the metal shell 1. Then, an opening edge on the rear end of the metal shell 1 is swaged inwardly toward the packing 60 (and the projecting portion 2e serving as a swaging support portion). As a result, a swaged portion 1d is formed and the metal shell 1 is fixed on the insulator 2.
A gasket 30 is inserted at a base end of the thread portion 7 of the metal shell 1. The gasket 30 is a ring-shaped part formed by bending a metal plate material such as carbon steel and is deformed such that it is compressed and crushed in the axial direction thereof between a flanged gas seal portion if on the side of the metal shell 1 and an opening edge of the tapped hole when the thread portion 7 is screwed into a tapped hole of a cylinder head, thereby sealing a gap between the tapped hole and the thread portion 7.
FIG. 2 is an explanatory diagram illustrating an exemplary process of swaging and fixing the metal shell 1 on the insulator 2 (ground electrode 4 is omitted). First of all, for the metal shell 1 shown in FIG. 2(a), as illustrated in FIG. 2(b), the insulator 2 is inserted through an insertion opening 1p (a swaging target portion 200 to be the swaged portion 1d is formed) at the rear end of the metal shell 1, where the center electrode 3, the conductive glass seal layers 16 and 17, the resistor 15 and the terminal metal piece 13 are previously installed in the through hole 6 of the insulator 2. The insertion of the insulator 2 allows an engagement portion 2h of the insulator 2 and an engagement portion 1c of the metal shell 1 to engage with each other through a plate packing member 63.
After that, as illustrated in FIG. 2(c), the linear packing member 62 is arranged in the inside of the insertion opening 1p of the metal shell 1. The filled layer 61 such as talc is formed, and furthermore the linear packing member 60 is arranged. Then, the swaging target portion 200 is swaged, by using a swaging mold 111, to an end face 2n of the projecting portion 2e as a swaging support portion through the linear packing member 62, the filled layer 61, and the linear packing member 60. As a result, the swaged portion 1d is formed as illustrated in FIG. 2(d). Moreover, the metal shell 1 is swaged to be fixed to the insulator 2. Here, not only the swaged portion 1d but also a groove potion 1h (see FIG. 1) between the hexagon portion 1e and the gas seal portion 1f is deformed due to compressive stress at the time of the swaging. The reason is that the swaged portion 1d and the groove potion 1h are thinnest and thus tend to be deformed in the metal shell 1. It should be noted that the groove potion 1h may be referred to as a “thin portion”. After the process illustrated in FIG. 2(d), the spark discharge gap g is formed by bending the ground electrode 4 toward the center electrode 3. In this manner, the spark plug 100 illustrated in FIG. 1 is completed. It should be noted that the swaging process described with reference to FIG. 2 is cold swaging (refer to Patent Document 2). Thermal swaging (refer to Patent Document 3) also is applicable.
Citation List
Patent Documents
Patent Document 1: JP-A-2002-184552
Patent Document 2: JP-A-2007-141868
Patent Document 3: JP-A-2003-257583
Patent Document 4: JP-A-2007-023333
Patent Document 5: JP-A-2007-270356
Problems to be Solved by the Invention
According to the above-mentioned related art (Patent Document 1), an electrolytic chromate processing, which allows 95% or more by mass of chromium component of a chromate layer to be trivalent chromium, is performed. Its object is to substantially eliminate hexavalent chromium in order to achieve reduction of environmental burdens and improve corrosion resistance to salt water (i.e. salt resistance).
However, as described above, the swaging process causes not only large deformation but also high residual stress in the swaged portion 1d and the groove potion 1h. Therefore, corrosion resistance in these potions is an important issue. That is, the swaged portion 1d and the groove potion 1h are characterized by having high residual stress due to the swaging deformation. In particular, in a case where the thermal swaging is used, textural variation due to heating causes increase in hardness. At such the position where the hardness is high and the high residual stress exists, stress corrosion cracking may be caused. The inventors of the present application have found that not only the salt resistance but also stress corrosion cracking resistance is an important issue particularly with regard to the swaged portion 1d and the groove potion 1h of the spark plug. Such a problem is conspicuous particularly in a case where a metal shell made from a material containing a large amount of carbon (for example, carbon steel containing carbon of 0.15% or more by weight) is used. This problem is conspicuous also in a case where the thermal swaging is used as the swaging process.
An object of the present invention is to provide a spark plug that is excellent not only in the salt resistance but also in the stress corrosion cracking resistance.